In a cellular communications network, user equipment (UE) (such as mobile telephones, mobile devices, mobile terminals, etc.) can communicate with other user equipment and/or remote servers via base stations. LTE systems include an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC) network (or simply ‘core network’). The E-UTRAN includes a number of base stations (‘eNBs’) for providing both user-plane (e.g. Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and PHYsical (PHY) layers) and control-plane (e.g. Radio Resource Control (RRC)) protocol terminations towards the UE.
Recent developments in communication networks have seen increased deployment of so called ‘small’ cells operated by Low Power Nodes (LPNs), such as pico eNBs, femto eNBs, home eNBs (HeNBs) or the like, which cells have a smaller coverage area than existing macro cells operated by a higher power (regular) macro base station. Networks comprising a number of different cell types, for example a network comprising a macro cell and a femto cell, are referred to as Heterogeneous Networks, or HetNets. In the following description the term base station is used to refer to any such macro base station or LPN.
In a related art, a mobile telephone is configured to communicate via one base station (using an associated radio link). However, in a study on small cell enhancements for E-UTRA and E-UTRAN (3GPP technical report (TR) no. 36.842 (V12.0.0), the contents of which are incorporated herein by reference), a so-called ‘dual connectivity’ functionality was introduced to improve, for example, the coverage of high data rates for user equipment, temporary network deployment, cell edge throughput and/or to increase system throughput. The dual connectivity feature established techniques for compatible mobile telephones (and other user equipment) to communicate with multiple network points, substantially simultaneously. Specifically, this ‘dual connectivity’ functionality refers to an operation mode where a given mobile telephone (operating in RRC CONNECTED mode) consumes radio resources provided by at least two different network points (e.g. two or more base stations). Typically, one of the network points involved in the dual connectivity functionality is a macro base station and the other network point (or a plurality of network points) comprises a low power node (or plurality of low power nodes).
Each network point involved in the provision of dual connectivity for a mobile telephone may assume a different role. One of the network points may be referred to as a master base station (MeNB) and each one of the other network points may be referred to as a secondary base station (SeNB). Typically, the various secondary base stations involved in the provision of dual connectivity are coupled (to the MeNB and hence the core network) via a so-called non-ideal backhaul. Further, in a dual connectivity scenario, one of the base stations (the MeNB) routes control plane signalling to the core network via an associated interface (e.g. the S1 interface), regardless of whether or not the other base station is also connected to the core network for user plane communication (e.g. to a serving gateway).
The MeNB/SeNB roles do not necessarily depend on each base station's capabilities/type (e.g. power class) and may be different for different mobile telephones (even when using the same base stations).
In accordance with the dual connectivity functionality, a mapping between the mobile telephone's radio (communication) bearer(s) and the base stations may be realised as follows:                Radio Bearer served by the MeNB only (MeNB-specific bearer);        Radio Bearer served by the SeNB only (SeNB-specific bearer); and        Radio Bearer served by MeNB and SeNB (split bearer).        
3GPP technical specification (TS) 36.314 (V11.1.0) defines a number of layer 2 (L2) measurements that may be performed by the base stations. Such measurements include, for example, one or more of:                measurement of physical resource block (PRB) usage (i.e. to measure usage of time and frequency resources);        measurement of received random access preambles;        measurement of number of active UEs;        measurement of packet delay;        measurement of data loss (i.e. to measure data packets that are dropped due to congestion, traffic management, etc.); and        measurement of scheduled Internet Protocol (IP) throughput.        
A base station can calculate downlink packet delay based on appropriate ‘arrival’ time point measurements (or ‘time stamps’) provided by the base station's PDCP entity (e.g. specifying the point in time when a particular data packet was received at the PDCP layer from upper layers) and ‘receipt’ time point measurements (or ‘time stamps’) provided by the base station's MAC entity (e.g. specifying the point in time when a particular data packet was confirmed to have been successfully received by the UE). The packet delay may be calculated for a single packet (e.g. as the time difference between the two time stamps associated with that single packet), or it can be calculated as an average value (e.g. the sum of time differences between respective pairs of arrival and receipt time stamps over the number of data packets).
The inventors have realised that, in the split bearer scenario, difficulties arise in performing packet delay measurements by the base stations (both the MeNB and the SeNB) when the PDCP and MAC functionalities (for the same bearer, or for different bearers) reside in different base stations.
Specifically, because the PDCP and MAC functionalities (for a given bearer) may reside in different base stations (e.g. the PDCP functionality may reside in the MeNB and (at least part of) the MAC functionality may reside in the SeNB—although part of the MAC functionality may reside in the MeNB), it may be impossible to calculate and report the packet delay measurements. In more detail, since the calculation of downlink packet delays is based on input (e.g. a respective time stamps) provided by both the PDCP layer and the MAC layer, when these layers are provided by different base stations (for split bearers), it may be impossible to calculate and report the packet delay measurements.
Further, when the calculation of the packet delay is based on averaging the respective packet delays for a plurality of data packets belonging to different communication bearers, the calculation can only use time stamp pairs of the non-split bearers thus it may not give an accurate picture of the actual packet delay for the split bearers (which would be most probably higher due to the involvement of at least two base stations rather than a single base station in case of non-split bearers).